The present invention relates to a method for transmission type photometric measurement of liquid samples for the purpose of determining the concentration thereof and in particular to a method for the transmission type photometric measurement of liquid samples contained in a series of cells arranged in a cell carrier and successively conveyed by the latter into the path of a light beam which is directed onto a radiation detector, and for use in a spectrophotometers and an automatic analysis apparatus.
Transmission type photometric measurements are normally made by simply directing a light through a cell cuvette containing a fluid sample to be analyzed. A portion of the light beam is absorbed by the sample and the remaining portion passes through the cuvette to photodetector means for measurement. By comparing the measurement obtained from the sample with a measurement obtained from a control fluid the concentration of the analyzed sample can be calculated.
The total absorbance of the sample is known to be composed of the sum of the absorbances of the sample and a thermostaticized bath or the like.
A major source of error in the transmission type photometric apparatus lies in imperfections in the glass or plastic cuvette. Very minor variations in a cuvette will substantially affect measurements made by the photodetector means. The variety of the light passageways through the cuvette is one of the above-mentioned variations.
In order to compensate for the above-mentioned problem, highly accurate photometric measurement method utilizes optically ground and polished glass or quartz to form the square-shaped cuvettes having at least two sides which are parallel surfaces of extremely high uniformity. However, such cuvettes are very expensive to manufacture, and have the possibility of contamination between test samples due to improper cleaning. For this reason accuracy is normally sacrified by using conventional cylindrical cuvettes resembling test tubes in transmission type photometric apparatus. There are changes in passageways of light through the cylindrical cuvette as the light beam laterally traverses the cross-sectional plane of the cuvette. When photometric apparatus is employed which passes light directly through the cylindrical cuvettes to a photodetector, substantial variation in light transmission results as the positions of photometric measurement are changed laterally in the cross-sectional plane of the moving cuvette.